1. Field of the Invention
The invention relates to an exhaust silencer device for an internal combustion engine. In particular, the invention relates to an exhaust silencer device for an internal combustion engine that is required of a silencing effect in a low rotation speed region of the engine.
2. Description of the Related Art
Lately, in a vehicle, such as a motor vehicle or the like, that is provided with an internal combustion engine, fine and delicate electronic control is performed in order to improve fuel economy, so that the vehicle travels with low throttle opening degree and low engine speed in an increased number of situations. Under such circumstances, in exhaust silencer devices for internal combustion engines, an arrangement is provided in order to meet a high requirement for the silencing performance against an unpleasant exhaust sound of a low frequency. That is, separately from a main muffler that silences exhaust sound from an internal combustion engine, a sub-muffler having a smaller muffler volume than the main muffler is provided so as to supplement the amount of silencing or improve the resonance characteristic of an exhaust path.
A known related-art exhaust silencer device of this kind for an internal combustion engine is, for example, an exhaust silencer device which has, in an exhaust path, a sub-muffler and a main muffler positioned downstream of the sub-muffler, and in which the main muffler has a multi-hole inner pipe that forms an exhaust passage and has many holes, an elongated casing that houses and surrounds the multi-hole inner pipe, and a sound absorbing material that is provided in a cylindrical space between the multi-hole inner pipe and the casing (see, e.g., Japanese Patent Application Publication No. 63-61519 (JP-A-63-61519), and Japanese Patent Application Publication No. 63-110641 (JP-A-63-110641)).
Besides, a device in which a main muffler is disposed in a front side and a rear muffler is provided in a rear end side of an exhaust pipe is also known (see, e.g., Japanese Patent Application Publication No. 2006-29224 (JP-A-2006-29224)).
Also known are a device that adopts as a sub-muffler a double resonance tube which has a resonance hole and a resonance chamber that communicates with the resonance hole (see, e.g., Japanese Utility Model Application Publication No. 61-190413 (JP-U-61-190413)), and a device in which an intermediate portion of the exhaust pipe has a double resonance tube structure so as to reduce the size of the main muffler (see, e.g., Japanese Patent Application Publication No. 2005-105918 (JP-A-2005-105918)).
In the foregoing related-art exhaust silencer devices for internal combustion engines, however, when the position of the downstream-side muffler is near an outlet of the exhaust pipe (exhaust opening) as shown in FIG. 8A, a sufficient silencing performance against the exhaust sound of a low frequency range near the idling vibration range cannot be achieved. Specifically, the exhaust passage length between the main muffler and the sub-muffler is long, and therefore, the air column resonance that occurs in that interval is of a very low frequency (e.g., in the vicinity of 30 Hz to 50 Hz), so that the exhaust sound deteriorates. Despite this, the muffler is disposed at a position of low sound pressure and low particle velocity that is near a node of the resonance mode as shown in FIG. 8B. Therefore, the silencing efficiency is low. Therefore, the muffler volume cannot be restrained, so that it is difficult to make the device compact or reduce the costs.
On the other hand, in the case where the downstream-side muffler is apart from the outlet (exhaust opening) of the exhaust pipe, and exhaust pipes having substantially equal piping lengths are present on two sides of the downstream-side muffler, there occurs coincidence between resonance frequencies of the exhaust pipes on the two sides, giving rise to a problem of exciting an exhaust sound of a relatively high frequency (e.g., in the vicinity of 150 Hz to 180 Hz).
Besides, due to the understanding that the muffler with a sound absorbing material is suitable for the silencing of relatively high-frequency sound and that the larger the muffler volume, the higher the silencing effect, a muffler with a sound absorbing material has not been effectively utilized to silence the aforementioned low-frequency exhaust sound. In this respect, too, the muffler volume cannot be restrained, so that it is difficult to make the device compact or reduce the costs.